1. Field of the Invention
The present invention relates to a fuel cell system and more particularly to a fuel cell system using a proton exchange membrane as an electrolyte.
2. Description of the Prior Art
proton exchange membrane fuel cell comprises a proton exchange membrane (PEM) between two electrodes, that is, a cathode to which an oxidizing gas is supplied and an anode to which fuel gas is supplied. The PEM acts as an electrolyte and transports therethrough hydrogen ions obtained at the anode of the fuel cell toward the cathode, in the form of protons (H+). Each of the electrodes comprises a catalyst layer deposited on a porous base member through which the reactant gas is supplied. Mounted externally of each electrode is a separator or connector plate with grooves permitting the reactant gas to be introduced into the electrode at a constant flow rate. Excess gas which has not been consumed by the fuel cell reaction is exhausted to the open air through the grooved separator. The electricity generated by the energy conversion reaction at the anode is collected at the electrode porous base member and transported outside of the fuel cell system through the separator. In actual application, the system includes a plurality of fuel cells which are stacked in series with the separator being interposed between adjacent fuel cells.
Since the fuel cell generates heat in correspondence to the electric power generated, a fuel cell stack 100 usually includes cooling plates 103 between fuel cells 101, 101 at predetermined intervals, as shown in FIG. 9. Each cooling plate has a passage for a cooling medium such as air and water to prevent excessive overheating of fuel cells 101 in operation.
The proton flow becomes hydrated when being transferred through the PEM electrolyte, so that the PEM tends to be dehydrated as the fuel cell reaction proceeds. The PEM must always be properly humidified to prevent decrease of ion-conductivity and energy conversion efficiency. In the conventional designs, hydrogen gas is humidified by suitable means which, in turn, humidify the PEM as the hydrogen gas is supplied to the anode.
Various attempts have also been proposed to humidify the air to be supplied to the cathode. Since the cathode of the fuel cell in operation has been heated to 80° C. for example, the room temperature air should be preheated by a humidifier so that its vapor content becomes consistent with the ambient vapor condition of the cathode. Such a humidifier, that is required to have a water supplying function and an air preheating function, can not be simple in construction.
In Japanese patent un-examined publication No. 7-14599, there is provided a water injection nozzle for injecting a necessary quantity of water into an air introducing pipe through which air is supplied to the cathode of the PEM fuel cell. Since the nozzle is located upstream of a compressor, liquid water injected from the nozzle is evaporated when subjected to heat generated by the compressor. Thus, the cathode is humidified by vapor, not by liquid water.
In the fuel cell system of Japanese patent un-examined publication No. 9-266004, a discharge gas from the anode containing hydrogen gas which has not been consumed during the anodic reaction is introduced into the cathode where the unconsumed hydrogen in the discharge gas is combusted with oxygen to generate water, which well humidifies the PEM electrolyte. In this system, there is no need to install a humidifier for humidifying the air to be supplied to the cathode.
During operation of the fuel cell system, electrons produced at the anode are moved to the cathode where they react with oxygen in the air or any other oxidizing gas supplied thereto to produce water. Accordingly, in accordance with the conventional practice in the art, there is a greater need to humidify the hydrogen gas to be supplied to the anode, than at the cathode where water can at least partially be self-sustaining.